1. Technical Field
The embodiments herein generally relate to microelectromechanical systems (MEMS), and, more particularly, to piezoelectric MEMS actuators.
2. Description of the Related Art
MEMS piezoelectric actuators are the basic building blocks for complex electromechanical systems such as radio frequency (RF) MEMS, optical switching, robotics, and many more applications. Generally, piezoelectric MEMS actuators significantly outperform the present standard—electrostatic MEMS actuators, at significantly smaller sizes, power consumption, actuation voltages, and displacement ranges. Moreover, piezoelectric MEMS devices typically permit extremely large displacements; approximately hundreds to thousands of microns, at very low voltages.
Conventional piezoelectric MEMS actuators are positive vertical unimorph actuators 1 as depicted in FIGS. 1A through 2. This MEMS technology, an adaptation of a decades old macro-scale design, has been around for over a decade. At the macro-scale, a large number of piezoelectric actuator designs have been developed and utilized. Many designs are based upon or are derivatives of the following basic technologies: unimorph and bimorph benders, stack actuators, externally frequency leveraged actuators like the Inchworm actuators, and externally kinematically leveraged actuators like the “Cymbal” & “Moonie” flextensional actuator designs. These macro-scale actuators can typically be utilized in an arbitrary orientation depending largely on the application. However, integrated MEMS devices do not share this characteristic and often the degree of freedom of the actuator has a dramatic impact on its design. For example, the dominant MEMS transduction technology, electrostatic MEMS, utilizes two very different designs for out-of-plane and in-plane actuation. Interdigitated “comb drive” actuators provide excellent capability for in-plane operation while “parallel plate” designs are typically employed for out-of-plane operation. The differences in these designs are dramatic. Only recently have attempts been made to design in-plane or lateral deflecting piezoelectric MEMS actuators.
A conventional piezoelectric unimorph actuator 1 is depicted in FIGS. 1A and 1B. The actuator 1 bends due to a voltage applied across the electrodes 2, 3, the piezoelectric effect, and a particular mechanical asymmetry of the structure of the actuator 1 relative to the structure's piezoelectric layer 4. The piezoelectric unimorph actuator 1 bends due to a piezoelectrically induced bending moment acting about the neutral plane (or planes). Fundamentally, there are two parameters that dictate the direction of motion of a piezoelectric cantilevered unimorph actuator 1, the sense of the strain within the piezoelectric layer 4 (sense of piezoelectric equivalent force component of the bending moment) and the relative position of the geometric mid-plane of the piezoelectric layer 4 with respect to the neutral plane or axis of the actuator 1 (sense of the moment arm of the bending moment). For out of plane (x-y) actuators, the relevant neutral plane is the x-y neutral plane. A structural dielectric layer 5 provides the necessary asymmetry for a non-zero moment. Moreover, a substrate (not shown) may be used to anchor one end of the actuator 1.
As illustrated in FIG. 2, for any three-dimensional object, there are three orthogonal neutral planes (axis); x-y, x-z, and y-z. The neutral plane (axis) is the location within the structure where there is equal contribution to structural stiffness (resistance to deformation) on either side of the plane (axis) and under pure bending, is the location of zero strain along the axis normal to this plane. If the strain field of the piezoelectric layer 4 is asymmetric about any of these neutral planes (axis), then it will contribute a component of a bending moment that acts upon the composite structure. For a vertical piezoelectric unimorph actuator 1, the piezoelectric strain field should be symmetric about the x-z and y-z neutral planes (axis). It is the x-y neutral plane (axis) that is relevant.
Piezoelectric materials deform (strain) when in the presence of an applied electric field. This behavior is due to the electric field induced atomic displacements within the crystalline unit cell of a piezoelectric material 4. These displacements cause the geometric distortion of the unit cell, and consequently, of the piezoelectric material 4 on the macroscopic scale as well. FIGS. 3A and 3B depict the ferroelectric unit cell of Lead-Zirconate-Titanate (PZT). FIG. 3A illustrates the high temperature non-piezoelectric form of PZT with a cubic unit cell. FIG. 3B depicts the low temperature piezoelectric form of PZT with a tetragonal unit cell. The central atom 12 of the tetragonal unit cell (either Ti or Zr) is displaced from the center of the unit cell. This vertical displacement represents the poled state of the material and is conditioned with an applied electric field. Multiple orientations or phases are possible for PZT; for simplicity, only the tetragonal phase is illustrated.
In FIGS. 3A and 3B, atoms 10 are Pb, atoms 11 are O, and the central atom 12 is either Zr or Ti; which is typically in a 52/48 compositional ratio. When an electric field is applied to the material that displaces the central atom 12 in the positive direction (arrow 13); the tetragonal unit cell is distorted further and a net elongation along the long axis 15 relative to the poled configuration occurs. When small electric fields are applied that displaces the central atom 12 in the negative direction; the tetragonal unit cell contracts along the long axis 15. Once the electric field exceeds the coercive filed of the material, the central atom 12 continues to displace in the downward direction (arrow 14) and the tetragonal unit cell experiences a net elongation again along the long axis 15.
FIGS. 4A and 4B illustrate the polarization/electric field plot of a ferroelectric material. This hystersis loop illustrates the relationship between polarization within a ferroelectric material and the applied electric field. Point 16, 18 where the loop intersects the field axis is the value of the coercive field. The coercive field is the electric field value required to cancel the internal remnant polarization of the ferroelectric material. Point 17, 19 where the loop intersects the polarization axis is the value of the remnant polarization. The remnant polarization is the measure of residual polarization remaining in the ferroelectric once the applied field has been removed. For small values of an applied electric field, the central atom 12 (of FIG. 3B) displaces positively or negatively from its poled position, depending upon the polarity of the field. For the scenario illustrated in FIGS. 3A and 3B at small electric field values, a positive displacement of the central atom 12 creates a net elongation (long axis 15) of the unit cell while a negative displacement creates a net contraction (long axis) of the unit cell relative to the poled unit cell orientation. However, for applied electric field values that exceed the value necessary to displace the central atom 12 back to the unit cell mid-plane (i.e. the coercive field), the central atom 12 will continue to displace in the direction of the applied field. Once this occurs, the unit cell will no longer experience a net contraction along the c axis, relative to the poled orientation, and instead will experience a net elongation along the c axis. This is due to the mirror symmetry of the unit cell about its mid-plane. Thus for large electric fields applied to the piezoelectric material, only a single sense of the piezoelectric strain is possible and therefore is independent of applied field polarity. As can be seen in FIGS. 3A and 3B, the sense of piezoelectric strain at high field strengths is in-plane contraction and out of plane (long axis) elongation.
This high field condition is rarely encountered in bulk ferroelectric/piezoelectric materials because the material thicknesses are so large. The behavior of piezoelectrics at large fields is not commonly understood in the MEMS community. However, for MEMS ferroelectric/piezoelectric actuators, this condition is typically encountered at small voltages (2-3V). The in-plane contraction of the piezoelectric material at large fields gives a negative sense to the piezoelectric equivalent force. The standard composite stack (FIGS. 1A and 1B) gives a positive sense of the moment arm. At small fields, with the appropriate polarity, a standard piezoelectric MEMS actuator 1 (FIGS. 1A and 1B) will deflect downward. However, as the voltage increases to a value near the coercive field, the actuator 1 will switch directions and will then bend upward. As the field strength is increased further, the actuator 1 will continue to bend upward. If the opposite polarity voltage is applied, the actuator 1 will bend downward for small voltages less than the coercive field. Again, as the voltage reaches a value greater than the coercive field the actuator 1 bends upward. Therefore, the standard piezoelectric MEMS unimorph actuator 1 is typically unable to attain large negative (into the plane) deflections.
Only recently have attempts been made to design in-plane or lateral deflecting piezoelectric MEMS actuators, “recurve” actuator technology developed by Ervin and Brei (Ervin, et al., “Recurve Piezoelectric-Strain-Amplifying Actuator Architecture”, IEEE/ASME Transactions on Mechatronics, Vol. 3, 293-301 (1998), the complete disclosure of which, in its entirety, is herein incorporated by reference) This actuator design can achieve large displacements but is limited to small force generation per unit area. J. Cheong developed MEMS flextensional actuators based on a buckled beam design; however this approach is severely limited to very small deflections (Cheong, J., “Design, fabrication, modeling, and experimental testing of a piezoelectric flextensional microactuator,” Ph.D. thesis, Department of Mechanical and Nuclear Engineering, Penn State University, 2005, the complete disclosure of which, in its entirety, is herein incorporated by reference). N. Conway and Kim have developed true in-plane piezoelectric MEMS actuators using a stroke amplification scheme. (Conway et al., “Large-Strain, Piezoelectric, In-Plane Micro-Actuator,” IEEE MEMS 2004, the complete disclosure of which, in its entirety, is herein incorporated by reference). This design is intended to be flextensional, utilizing the in-plane strain of the piezoelectric material and a kinematic mechanism to provide limited amplification of this naturally small deflection. The design features a full top electrode and is intended to suppress bending action of the device altogether. These existing lateral piezoelectric MEMS actuators have utilized undesirable methods for increasing piezoelectric actuator stroke length, either in terms of lost work efficiency, or in chip area required to leverage the basic actuation stroke and are generally subject to variation in performance due to residual stress deformation.
There are a number of applications for lateral piezoelectric MEMS actuators, including RF MEMS and millimeter-scale robotics that require greater deflection, force, and work per unit area than the current state of the art can provide. RF MEMS devices can benefit from greater contact forces and free displacements from lateral piezoelectric MEMS actuators and greater deflection, force, and work per unit area per unit power actuators can enable millimeter-scale robotics. Therefore, there exists a need for lateral deflecting piezoelectric MEMS actuators with improved deflection, force, and work per unit area performance.